Climate change and anthropogenic pollution are leading to the eutrophication of aquatic ecosystems, creating favorable conditions for the proliferation of cyanobacteria. Some cyanobacteria species produce cyanotoxins, including microcystins (MCs), which can contaminate irrigation water and thus negatively affect crop productivity, and consequently food security and human health. To mitigate these impacts, lichen extract represents a promising source of bioactive compounds for sustainable agriculture. These extracts are known to stimulate plant growth, improve nutrient uptake, and increase tolerance to environmental stress. In this context, the potential of aqueous extract of Pseudovernia furfuracea was evaluated with regard to its effect on the growth and stress resilience of radish (Raphanus sativus L.) grown hydroponically and exposed to MCs. Two concentrations of MCs (10 and 40 µg/L) and a lichen extract (0.1 and 1 g/L) were applied individually or in combination for 45 days. Exposure to MCs alone inhibited growth, reduced chlorophyll and protein levels, and induced oxidative stress, as evidenced by elevated levels of malondialdehyde (MDA), polyphenols, and sugars, as well as increased activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). In contrast, the combined application of the lichen extract and MCs mitigated these adverse effects, restoring growth parameters, chlorophyll, protein, and sugar levels, while strengthening antioxidant defense systems. The results obtained highlight the effectiveness of P. furfuracea extract as a natural biostimulant capable of improving radish growth and its tolerance to MC-induced stress.